The present invention relates to a silver halide photographic material, in particular, one which is improved in spectral sensitivity, keeping quality and resistance to blackening under pressure.
Silver halides have an inherent light absorption band which is usually within the blue to ultraviolet regions but not within the green or red region. Thus, silver halides inherently lack sensitivity to green and red light, or their sensitivity to these lights is too small to achieve high efficiency. To cope with this situation, it is common practice in the art of silver halide photographic material to incorporate in photographic emulsions those organic dyes which have a light absorption band in the visible range (the infrared range in the case of infrared light-sensitive materials) so that their sensitivity will be extended to the visible or infrared range by having those organic dyes adsorbed on the silver halide grains. This technique is referred to as "spectral sensitization" and the dyes used are called "spectral sensitizing dyes" or simply "spectral sensitizers". The demand for higher sensitivity in modern photographic materials may be translated as the requirement for improvement in the sensitivity achieved by "spectral sensitization" or "spectral sensitivity".
The production of silver halide emulsions generally proceeds through the steps of forming silver halide grains, physical ripening, desalting, chemical sensitization, etc. Spectral sensitizers may be added at various stages of the production process, and three approaches have been taken in the prior art. According to the first approach, spectral sensitizers are added after chemical sensitization and before coating, as described in U.S. Pat. No. 4,425,426, prior to chemical sensitization, or during chemical sensitization. In the second approach, spectral sensitizers are added during the growth of grains (during physical ripening) as described in U.S. Pat. Nos. 2,735,766, 3,628,960, 4,183,756 and 4,225,666, as well as in No. JP-A-55-26589 and No. JP-A-58-184142 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). The third approach consists of adding spectral sensitizers both in the latter period of grain growth where the change in the shape of grains is substantially absent and after grain formation but before the desalting step, as described in Nos. JP-A-61-103149 and 61-196238.
The first approach, however, has had the problem that on account of weak adsorption of dyes onto silver halides, the technique is only applicable to the case where dyes capable of efficient spectral sensitization and having high adsorption power are to be used. The second approach which involves adding dyes in the process of grain formation is also disadvantageous in that the dyes will be adsorbed on the surfaces of growing grains, thereby interfering with normal crystal growth.
In spectral sensitization of silver halide emulsions, more than one dye is normally used to make emulsions having sensitivity in a certain wavelength range (say, green light), but with such systems using combined dyes, it is impossible to attain optimum spectral sensitization by merely increasing the absorption power of the dyes. As is well known, the interaction between dyes and silver halide grains is not the sole factor that govern systems that use several dyes in combination and the relationship with the interaction between different dyes will influence the efficiency of spectral sensitization by a great degree (see, for example, Chapter 10 of "The Theory of Photographic Process", 4th Ed., Macmillan Publishing Company, 1977). Therefore, optimum spectral sensitization requires the choice of not only optimum relationship between dyes but also optimum adsorption between dyes and the surfaces of silver halides. According to the results of the study conducted by the present inventors, the second and third conventional methods of adding spectral sensitizers involve considerable difficulty in improving the efficiency of spectral sensitization by controlling dyes so that they will be adsorbed onto silver halide grains in an optimal state. It was also found that these methods caused a negative effect in that resistance to blackening under pressure deteriorates.
Japanese Patent Application No. 62-119381 describes a technique of spectral sensitization that consists of adding dyes during the desalting step. This technique solves to some extent the problems described in the previous paragraph in association with the second and third approaches but on the other hand, the power of adsorption between dyes and silver halide grains is still insufficient to provide satisfactory improvement in resistance to blackening under pressure after storage in a hot and humid atmosphere.